In semiconductor manufacture microlithography is used in the formation of integrated circuits on a semiconductor wafer. During a lithographic process, a form of radiant energy, such as ultraviolet light, is passed through a mask or reticle and onto the semiconductor wafer. The reticle contains opaque and transparent regions formed in a desired pattern. A grating pattern, for instance, may be used to define parallel spaced conducting lines on a semiconductor wafer. The ultraviolet light exposes the reticle pattern on a layer of resist formed on the wafer. The resist is then developed for removing either the exposed portions of resist for a positive resist or the unexposed portions of resist for a negative resist. The patterned resist can then be used during a subsequent semiconductor fabrication process such as ion implantation or etching.
As microcircuit densities have increased, the size of the features of semiconductor devices have decreased to the submicron level. These submicron features may include the width and spacing of metal conducting lines or the size of various geometric features of semiconductor devices. The requirement of submicron features has necessitated the development of improved microlithographic processes and systems. As an example, phase shifting microphotolithographic processes use phase shifting reticles to phase shift the exposure radiation at the edges of a pattern to increase the image contrast. Other sub-micron microlithographic processes include e-beam writing and holographic lithography.
Different microlithographic illumination systems have also been recently developed for improving the resolution and depth of focus of fine microlithographic patterns in the half to sub half micron region. In general, the two types of illumination that are most useful for sub micron microlithography are partially coherent illumination and off-axis illumination. These different illumination systems illuminate the reticle in different ways altering the Fraunhofer diffraction pattern in the Fourier transfer plane. The Fraunhofer diffraction pattern or spectrum of an illuminated object is its Fourier transform. This "filtering" or modification of the transform pattern has a very significant effect on the resulting image.
Partially coherent illumination is concerned with spatial rather than temporal coherence and is achieved by under filling the entrance pupil in a Koehler type illumination system. Koelher illumination is where the exposure source is projected through a condenser lens and focused in an entrance pupil plane of the objective lens. This under filling of the entrance pupil is achieved by reducing the source diameter typically with aperture stops. This partially coherent illumination filters or blocks the diffracted light orders from the objects high frequency features, which increases the image contrast on the low and medium frequency features. This contrast improvement is achieved because the scattered low modulation diffracted light from the high frequency features is absent.
FIG. 1 illustrates a lithographic system 10 with partially coherent illumination. The partially coherent lithographic system 10 includes a source 12 of radiant energy such as UV light. The light from the source 12 passes through an aperture 14 and a condenser lens system 16. A reticle 18 (object plane) is positioned at the exit pupil of the condenser lens system 16 and is illuminated by the light beam emerging from the condenser lens system 16. Light passed through the aperture 14 is then focused into the entrance pupil 22 of an objective lens 24. The objective lens 24 focuses this light on the wafer 20 which is located at the image plane. The reticle 18 is formed of opaque and transparent areas for forming a desired pattern on the wafer 20. In FIG. 1, NA represents the numerical aperture of the objective lens 24, W represents the diffraction angle of the illumination beam and s represents the partial coherence value (i.e. % entrance pupil illuminated).
In off-axis or dark field illumination the illumination rays strike the object or reticle at an oblique angle. This is accomplished by blocking the normally incident zero order or undiffracted beam. For features smaller than the diffraction limited frequency as defined by the maximum diffraction angle accepted by the objective lens, only zero order or unmodulated light is transmitted through the lens. This unmodulated energy just adds to the modulated energy in the aerial image, reducing the image contrast. Off-axis illumination effectively blocks this light by shifting the zero order light to now be in the position of the maximum .+-.1 order diffracted beam accepted by the lens. This allows a larger .+-.1 order diffraction angle beam to be accepted by the lens. Image formation now occurs by the interference of two beams being the zero order and either the +1 or -1 diffracted beam. This is represented schematically in FIG. 1A. This effectively increases the resolution. The depth of focus is also significantly increased with this two beam image formation because there is less or theoretically no phase (optical path length) difference between the image formation beams in the defocus mode.
In theory, the image resolution for off axis illumination is two times the resolution of coherent illumination. The coherent illumination (i.e., illumination with normally (90.degree.) incident rays) resolution limit can be represented as:
RES.sub.cut-off =spatially coherent resolution limit ##EQU1## where NA is the numerical aperture of the objective lens, .lambda. is the illumination radiation wavelength, and w is the maximum acceptable diffraction angle. PA1 The partially coherent resolution limit ##EQU2## where s=sigma=degree of partial coherence PA1 Off-axis resolution limit.sub.cut-off ##EQU3## where .theta.=off-axis illumination incident angle on mask PA1 Off-axis resolution limit ##EQU4## w is the diffraction angle of a small opening "a" on the reticle given as: m .lambda.=asin (w) PA1 .lambda. is the actinic illumination wavelength; PA1 w is the diffraction angle
The off axis illumination resolution limit is given by:
When the source image is at the edge of the entrance pupil (i.e. .theta.=NA (1-s)) then the off-axis illumination (i.e. oblique rays illuminating object) resolution limit can be represented by doubling the above equation as:
where
Since 1989 there has been much published on the use of off-axis illumination for improving microlithographic performance. Some of these publications are listed in the Information Disclosure Statement filed herewith.
Although off-axis illumination results in improved depth of focus and resolution for microlithography, there are problems associated with this technique. One noted problem is the over exposure of the isolated features of a pattern. This situation occurs in a defocus mode and causes the projected exterior features of a repetitive pattern, or the exterior portions of features of the pattern, to degrade significantly from the dense line pattern. This condition is shown in FIGS. 2 and 3.
FIGS. 2 and 3, show two patterns projected onto a wafer in an off-axis illumination system during a defocus condition (i.e. non optimal z-position). In FIG. 2, the reticle pattern comprises a simple grating which includes parallel spaced solid lines 26 having a linewidth of LW. For a simple grating the linewidth LW also equals the space between the lines.
In the grating pattern of FIG. 2, the solid lines 26 are oriented generally perpendicular to the wafer flat. Because of the defocus condition, the exterior portions 28 of the solid lines 26 of the pattern projected onto the wafer (20 FIG. 1) have a degraded image profile. In addition, the exterior solid lines 30, 32 of the pattern are degraded. The top exterior line 30 is shown as having a curved profile and a linewidth that is much less than the desired linewidth LW. The bottom exterior line 32 is shown as having degraded exterior portions 28 and also degraded interior portions 29.
In FIG. 3, the solid lines 34 of a simple grating pattern are oriented generally parallel to the major flat of the wafer. Because of the defocus condition, the exterior portions 36 of the projected solid lines 34 have a degraded image profile. In addition, the exterior solid lines 38 and 40 of the pattern as a whole have a degraded curved profile substantially as shown. In addition, a width of the exterior lines 38 and 40 is less than a desired linewidth LW. At an extreme negative defocus conditions (e.g. 1.5 .mu.) the exterior linewidths 38, 40 (FIG. 3) or 30, 32 (FIG. 2) will vanish completely.
Such degraded profiles with off-axis or partially coherent illumination systems have been extensively documented in the art, (e.g., see the article "0.35 .mu.m Lithography Using Off-Axis Illumination", Luehrmann et al., SPIE-Optical/Laser Microlithography VI Conference, March 1993, San Jose, Calif.).
This condition is also known as the "proximity effect" or "proximity image degradation". In general, the proximity effect refers to the exterior line or space degradation of a dense line-space structure in the defocus mode. It is theorized that the degraded exterior features are in proximity to larger "low resolution" areas that are void of patterns. These low resolution areas scatter light in the defocus mode causing the image degradation. The low frequency modulation degradation is due to optical path length differences between the zero and the .+-.1 orders required for image formation. Scattered or stray actinic radiation from the low resolution areas reduces the image contrast in the high resolution areas, which significantly degrades the image quality of these adjacent features.
Due to this image degradation problem, the contrast, resolution and depth of focus for lithographic systems that employ partially coherent or off-axis illumination is significantly impaired. Depth of focus is the maximum range the image plane (e.g. wafer) can move along the z axis and still produce an acceptable image or photoresist pattern. Acceptance is typically defined as a critical dimension range (physical size of photoresist pattern) of .+-.10% about some target dimension. This acceptable photoresist pattern must also have sidewalls that are not degraded by more that 5 degrees and a resist thickness loss less than 10%. These qualities are all relative to the best focus image plane position for achieving a resist pattern.
In view of the these shortcomings of microlithographic systems that utilize partially coherent or off-axis illumination, it is an object of the present invention to provide an improved reticle pattern for microlithographic systems that utilize partially coherent or off-axis illumination.
It is another object of the present invention to provide an improved reticle pattern for semiconductor lithography that improves the resolution, contrast and depth of focus for features projected onto a semiconductor wafer.
It is yet another object of the present invention to provide an improved reticle pattern for lithography suitable for use with off-axis or partially coherent illumination and in which submicron exterior features of a dense line-space pattern (e.g. periodic grating) are protected from degradation in a defocus mode of the system.
It is a further object of the present invention to provide an improved method of semiconductor lithography in which differences in the linewidth dimension between interior, exterior, and isolated features in a submicron resist pattern are improved.
It is a further object of the present invention to provide an improved reticle pattern for off-axis illumination in semiconductor lithography that overcomes the proximity effect.
It is a still further object of the present invention to provide an improved method of lithography and an improved reticle pattern that are suitable for large scale semiconductor manufacture.